I was trying to go through all possible things that could cause the ECU to act as if there were more airflow than actually present.
The only bit left to consider was the brand new (and very expensive) EGR valve. If that was indicating it was open, it could cause the ECU to give more fuel to match the added air.
I swapped the motor unit on the top for the one off the old valve - and it now starts and ticks over without ann throttle. Fault valve unit.
I disconnected the battery for an hour to clear the settings from when it was messed up, then reconnected it and took it for a drive to try and let it set itself up.
It is still hunting / surging badly, especially with light throttle in traffic; a couple of times it just about gave me whiplash...
When stopping at traffic lights etc., it hunts between around 500 - 1100 revs for a while then settles at about 800-900 or so. It sometimes starts hunting again on its own.
I'm presently waiting for it to cool down enough to check the plugs again, to get an idea of the mixture.